[JENKINS-16993] Use "git log" to determine changed files.

The output of "git show" included commits that were not introduced since
the last build, so could falsely trigger build in the presence of
"includedRegions".

A benefit of not including extraneous commits is the size of the output
is reduced, and therefore less heap required.

This can still trigger extra builds as "-m" means the changes between all
legs of a merge commit are displayed.  However, without this, conflict
resolution changed as part of the merge commit would be skipped.